1. Field of the Invention
The present invention relates to an air conditioning apparatus, and more particularly to an air conditioning apparatus equipped with a heat exchanger which is suitable to control an amount of radiant heat from a condenser, and to a control method for the apparatus.
2. Prior Art
A heat exchanger used as a condenser for car air-conditioners was conventionally arranged by bending a perforated, extruded flat tube in the meandered or zigzag form, and disposing a number of fins between adjacent parallel portions of the zigzag tube. Because the above arrangement produces the increased pressure drops of a refrigerant flow, however, there has recently been practiced a heat exchanger comprising a number of heat conducting pipes arranged side by side between a pair of headers disposed parallel to each other for thereby reducing the pressure drops, as disclosed in Japanese Patent Unexamined Publication No. 63-3191 and Japanese Utility Model Unexamined Publication No. 64-22171 and No. 63-54690.
In the foregoing prior art, the heat transfer area of a heat exchanger is set to be capable of generating an amount of radiant heat necessary as a condenser even under a condition that the atmospheric temperature is high and the maximum cooling capacity is required in a refrigerating cycle. Thus, the heat transfer area of the heat exchanger is determined in anticipation of maximum load. Because the condenser is mounted in front of a radiator and cooled with the incoming air while an automobile is traveling, running conditions greatly change dependent on a vehicle speed. The temperature in compartments is also changed from about 40.degree. C. to about 20.degree. C. when starting up an air conditioner after an automobile has been parked in the open air in summertime, hence large fluctuations in heat load. Accordingly, a proper amount of refrigerant sealed off in the cycle is also greatly changed dependent on running conditions. In order to adjust that proper sealed-off amount, a receiver is installed on the outlet side of the condenser.
As the cooling load of the refrigerating cycle is reduced with the atmospheric temperature lowering, the condensing capacity of a heat exchanger is relatively enhanced by the combined effect of a reduction in the flow rate of refrigerant recirculating through the cycle system and a reduction in the atmospheric temperature for cooling the heat exchanger. As a result, especially under a condition of the low atmospheric temperature, the amount of refrigerant accumulating in the heat exchanger mounted as a condenser outside the compartment is increased, while the amount of refrigerant residing in the receiver, installed on the outlet side of the heat exchanger for adjusting refrigerant containment into the cycle, is decreased. This causes air bubbles to flow into an expansion valve, whereupon the cycle system causes a hunting and the refrigerating cycle fails to run normally. Overcoming that problem requires to enlarge the volume of the receiver and increase an amount of refrigerant sealed off. However, the cycle using R12 suffers from the problem of increasing an amount of refrigerant used which is under CFC regulations for the protection of earth environments. Also, use of an alternative coolant R134a or the like raises the problem of increasing an amount of expensive refrigerant used. Furthermore, in a refrigerating cycle system which includes a compressor of variable displacement type utilizing the pressure of delivery gas as a drive force for a capacity control mechanism, as described by way of example in SAE, Technical Paper Series 850040, 1985, the discharge gas pressure of the compressor is not raised as the condensing capacity of the heat exchanger is relatively enhanced under a condition of the low atmospheric temperature. This suffers from the problem that the capacity control can no longer work and an evaporator is frozen due to the excessive discharge flow rate.